Multiple sclerosis (MS) is a clinical neurological disease characterized by chronic inflammation, demyelination and gliosis of the central nervous system. It is the principal neurologic disease of individuals in early to middle adult life and afflicts as many as 350,000 people in this country. The available therapy for treating and preventing the progression of disease is limited. Recent evidence suggests that early therapeutic intervention may be important in decreasing disease activity. MS is mediated at least in part by T cells, macrophages and to a lesser extent B cells. Activation of T and B cells is dependent upon the interaction of the TCR/MHC as well as co-stimulatory molecules, including CD40-CD40L. CD40 and its ligand have been shown to play a critical role in the regulation of both humoral and cell-mediated immunity. Blockade of CD154 is effective in ameliorating the manifestations of several autoimmune diseases and thus has become an attractive therapeutic target. The interaction of CD40/CD40L has been demonstrated in both the experimental model of experimental allergic encephalomyelitis (EAE) as well as multiple sclerosis. In EAE, antibody blocking of CD40L prevents the progression of disease in both the monophasic and relapsing remitting experimental models. The overall objective of this clinical research trial is to determine whether antibody to CD40L can block the progression of enhancing lesions on MRI in those individuals with relapsing- remitting MS (primary outcome). Individuals (n=40) with clinically defined MS (EDSS 0-less than 3.5) will be evaluated for six months by monthly GdDTPA enhanced MRI scans and EDSS to determine disease progression. If eligible (greater than 2 new enhancing lesion 2mm or greater during the 6 month pretreatment phase), these individuals (n=20-24) will be treated with intravenous humanized anti-CD40L (IDEC-131). Patients will be treated for 6 months (total=8 infusions) with 5mg/kg antibody during which time they will continue to undergo monthly Gd enhanced MRI and EDSS evaluation(secondary outcome). We will determine the effect of therapy on MR spectroscopy metabolic, peaks for NAA, choline and creatine (absolute magnitude and ratios) in left and right hemispheric periventricular white matter voxels of interest (secondary outcome). Delay in cerebral atrophy will be evaluated by determining the change in parenchymal brain fraction. We will also develop a novel biological assay to determine efficacy of therapy(secondary outcome). MHCII-Ig fusion proteins that have an embedded MBP peptide will be used to identify MBP-reactive T cells in the peripheral blood of these patients. The polarity (Th1 and Th2) of the MBP-MHCII-Ig binding T cells will be determined as well as their activation status. The impact of in vivo anti-CD40L therapy on the frequency, phenotype and functional status of the MBP-MHCII-Ig binding T cells will be determined in vitro as well as T and B cell recall to tetanus toxoid. An additional secondary outcome will be to assess change in the cognitive impairment index in response to therapy. Neuropsychological variables sensitive to clinical trial outcomes in MS include measures of visual memory and mental flexibility. Follow up will be done with MRI and EDSS at 18 and 24 months after enrollment(secondary outcome). The study will will allow us to achieve a power of 85 percent (32 percent reduction in plaque burden).